A wide range of security screen systems are available for doors and windows. These systems typically attempt to secure a mesh within a frame by using conventional fasteners such as rivets, screws or glue, by using wedges, or by sing a deformation of the edges of the mesh.
A common screen system attaches the mesh to the frame by a simple fastener. Generally the mesh is attached to the outside wall of a frame with a screw, rivet or glue. However, the fasteners are vulnerable when the mesh is impacted or when an attempt is made to pry the mesh from the frame. The mesh is also semi rigid and may appear floppy. As the mesh is not taut it can extend or bulge outside of the perimeter of the frame. This can interfere with the normal operation of the product. For example, the mesh in a sliding door can rub or grind against the mullion of the glass door that is adjacent to it.
Fabrication of the product can also be difficult and time consuming because the mesh must be balanced in place during attachment of the fasteners to the frame. The mesh cannot sit flat on an assembly bench. The fabricator usually places packing under the mesh so that it sits in the final position and places weights on top of the mesh to attempt to keep the mesh flat. Once the mesh is fastened to the frame there is no possibility of tensioning the mesh. If glue is used, then the product must remain stationary until the glue sets. The fasteners grip only a very small edge of the mesh. This means that if the mesh is cut slightly too small, the retention of the mesh is weakened considerably as the fasteners will be too close to the edge of the mesh or, if glue is used, will not sufficiently cover enough surface area on the mesh. If the mesh is cut slightly too large, then the mesh will be unavoidably floppy.
Another common screen system involves a frame which has a channel section to receive the mesh. The mesh is placed into the channel of the frame and wedges are forced between one or both of the channel walls and the mesh. The wedges are typically made from a soft material such as PVC A similar principle uses a spline to retain a flyscreen in a frame channel.
These products overcame some of the weaknesses of the products that relied on fastening the mesh directly to the frame. Aesthetic appearance improved (because of the absence of unsightly fasteners), fabrication became easier (as the need to hold the mesh into a fixed position was removed) and the insertion of wedges allowed some tensioning of the mesh.
However, the flatness or tautness of the mesh could not be assured. The fabricator could not control the amount of tension on the mesh as the wedges were forced into position in the frame channel. Mesh is semi rigid and not uniform or consistent, and could have small waves of stress points. Without being able to control the tensioning of the mesh the inconsistency of the flat surface of the mesh often remained after inserting the wedges. For example, once the wedges were inserted into position into one channel of one frame there would be immediate stress on the mesh. It would be common to form stress diagonally, etc across the mesh. This often resulted in bubbles or waves in the mesh. If the mesh was cut too short the mesh would not be sufficiently retained in the channel.
If the mesh was cut too long, it would result in a floppy screen. This method also meant that the product could not be unassembled or adjusted. If the screen was made to the incorrect size, the product could not be unassembled and the components reused without significant damage.
Retention of the mesh to the frame is also weak when wedges are used. The wedge material relies on pressure or friction to retain the mesh to the frame. Due to the soft nature of the wedge materials the wedge could easily loose its grip on the mesh and release the mesh on impact. The assembly also relied on the strength of the channel wall which was an aluminum extrusion. Due to the low strength of aluminum, the channel walls are also prone to splay or spread apart on impact of the mesh and consequently loosen the grip on the mesh. The wedges sometimes pull out of the channel and the mesh can slip past the wedges.
Another screen system deforms the edge of the mesh and engages the edge with a corresponding portion of the frame. Typically the edge of the mesh is bent to form an angle or hook. This angle or hook is forced into the frames channel. Inside the frame channel is a tooth or some type of edge to engage the hooked portion of the mesh. Sometimes rather than engaging one wall of the channel a wedge is forced into the frames channel between one channel wall and the straight portion of the mesh so that the wedge rests over the top of the hook or bent portion of the mesh.
The impact strength of these screens is weaker than conventional fasteners. They are difficult to accurately fabricate as they rely on both perfectly cutting the mesh and bending the edge of the mesh. They cannot be adjusted for tension. Undercut mesh is impossible to correctly engage in the channel. Overcut mesh is floppy. In addition these systems rely on additional machinery such as a press brake to bend the edge of the mesh. This machinery is relatively expensive.
These products also have a corrosion potential as the mesh is typically stainless steel and it rests in an aluminum frame channel. The channel can easily retain water which can set off galvanic corrosion between the stainless steel mesh and the aluminum channel.